The present invention generally relates to hemapheresis apparatus and methods and particularly relates to apparatus and methods for the transmission of a positive gauge liquid pressure signal to a gas column from a pulsatile blood flow stream which is hydrostatically higher than the gas column, while maintaining the gas/blood meniscus.
In blood processing systems, it is usually necessary to monitor certain process stream pressures asceptically and continuously. This monitoring is typically done with a blood column to air column interface where the monitoring point is hydrostatically lower than the transducer. For example, there has recently been developed an instrument which provides for alternate blood collection and packed cell reinfusion cycles through a single needle, while simultaneously and continuously separating whole blood into packed cells and cell-free plasma. In such system, there is provided a hemapheresis instrument comprised of a series of microprocessor-controlled pumps, clamps, sensors and detectors on the face of the instrument, onto which a harness or tubing set is applied. The harness set includes a venepuncture needle, a reservoir, a separator and ancillary tubing for measuring pressures at various locations in the harness set after it is applied to the instrument face and during operation. In that system, the transmembrane pressure, i.e., the pressure across the filter membrane, is measured as a function of the pressure at the inlet port and which measurement is taken at an elevation below the pressure transducer. To effect the measurement, blood at the pressure monitoring location flows in a conduit, as in a manometer, and traps a column of air, the pressure of which is measured at the pressure transducer. To ensure asceptic conditions, a liquid or blood sensor is interposed between the blood/air interface of the conduit ahead of the pressure transducer such that, should blood be detected, the system can be shut down. This blood sensor, however, requires a tubing having sufficient cross-section to enable the sensor to generate a signal.
A system for generating platelet concentrate has been developed wherein it is desirable to monitor the pressure at the output of a separator, i.e., between the separator outlet and a peristaltic pump in such output line. Such system is described and illustrated in co-pending application Ser. No. 07/125/099, filed Nov. 25, 1987, of common assignee herewith. In such system, the pressure monitoring location is higher than the pressure transducer. It is important however, to maintain many of the features of the previously described system in this newer system. Consequently, for this hemapheresis system to transmit a pressure signal from a pressure monitoring location higher in elevation than the pressure measuring point while maintaining and air/blood interface, the system must be asceptic, must transmit air to an air column, the pressure of which could then be measured, must be compatible with a liquid sensor in the air column to preclude liquid or blood from reaching the pressure transducer, and must operate such that a wide range of both positive and negative pressures can be measured.
With reference to FIGS. 2A and 2B hereof, it was initially believed that simply running tubing, as in a manometer, from the elevated pressure monitoring point designated PMP to the lower liquid sensor LS and pressure transducer PT would enable the pressure of the pulsatile blood at the monitoring location to be measured while maintaining the air column between the air/blood interface and the pressure transducer. However, the standard tubing used, i.e., tubing having an interval diameter of 0.125 inches, allowed blood to flow along the side of the downwardly sloping portion of the tubing as indicated at 2 in FIG. 2A and fill the tubing at its lower U-bend. This trapped a pocket of air A behind the charge or slug S1 of blood. When the pressure was relieved, as frequently occurs in this process, this blood slug moved back up toward the process stream. On subsequent pressure increases, second and additional air slugs S2, S3, etc., as illustrated in FIG. 2B, would form and eventually the multiple sequential air/blood slugs formed in the tubing moved upline to the liquid sensor LS. That is, while the total volume of air in the line remained constant, it became dispersed between the blood slugs S enabling the leading blood/air slug S1 to finally reach the liquid sensor. Additionally, it was noted that the pulsations from the peristaltic pump broke down the meniscus in the downline of the tubing, allowing the blood to flow along the inside wall of the tubing thereby forming the air/blood slugs. Consequently, a substantial problem developed in efforts to monitor the pressure of the blood stream adjacent the output of the separator at an elevation above the pressure transducer.
The present invention is directed to apparatus and methods for transmitting the pressure signal from the monitoring location to a pressure transducer located at a lower elevation, while maintaining the gas/blood interface necessary to support the air column by which the pressure measurement is made. To accomplish this, it was recognized that a predetermined volume of gas had to reside in the upline tubing and the pressure transducer to enable the gas/blood interface to act as a piston compressing the air until dynamic equilibrium existed. Additionally, it was recognized that over the range of pressures which were to be measured, an excessive length of constant diameter tubing would be necessary to enable compression of the gas column. This also allowed blood to run down the downline and fill the trap or turnaround point thereby forming gas/blood slugs which, after several pressure cycles, would reach the liquid detector.
It was then recognized that the downline volume could be minimized to move the blood to the turnaround point after a much lower system pressure change. Thus, the downline tubing internal diameter was reduced, in comparison with the internal diameter (I.D.) of the upline, between the monitoring point to a location just beyond the turnaround point. Consequently, the downline would fill with blood with a very small pressure change. It was also recognized that such smaller diameter of the downline tubing enabled the meniscus to remain intact, even with the pressure pulses resulting from the peristaltic pump, and further that the internal diameter (I.D.) of the upline tubing could remain sufficiently large to accommodate the range of measurable pressure requirements of the liquid detector. Therefore, by providing a downline having a substantially smaller internal diameter than the upline, the meniscus is strengthened through an increase in the surface tension of the blood vis-a-vis the tubing, and the sensitivity of the liquid sensing detector is accommodated. Accordingly, the pulsatile blood pressure at the monitoring point, located at an elevation above the pressure transducer, may be transmitted to an air column at the pressure transducer, by using a smaller internal diameter downline connected with a larger internal diameter upline, the transition taking place at an elevation at least level with the turnaround point or higher on the upline tubing.
It was further discovered that a Y-connector at the juncture of the smaller internal diameter downline and the larger internal diameter upline may be used. In such system, the smaller pressure changes could be monitored while the meniscus remained intact as the blood moved into the downline. Numerous cycles of pressure demonstrated that this configuration also provided the required gas/blood separation. The Y-fitting configuration hereof is significant in that it enables the downline and upline of the harness sets to run parallel one to the other thereby facilitating manufacture of the harness set and application of the harness set to the instrument.
Accordingly, in a preferred embodiment of the present invention, there is provided apparatus for transmitting a positive gauge liquid pressure signal to a gas column from a pulsatile fluid stream at a pressure monitoring location hydrostatically higher than the pressure transducer, comprising a first conduit in communication at one end with the fluid stream at the pressure monitoring location for transmission of the fluid therethrough, and having a predetermined internal diameter of a size sufficient to maintain the meniscus of the fluid intact as the fluid flows through the first conduit. A second conduit is provided in communication at one end with the opposite end of the first conduit and in communication at its opposite end with the pressure transducer for containing the gas column, the second conduit having a predetermined internal diameter greater than the internal diameter of the first conduit. The first and second conduits are disposed such that a portion of the first conduit lies below the pressure transducer and substantially reverses direction before connecting with the second conduit whereby the pressure of the gas in the second conduit at the pressure transducer provides a direct measurement of the pressure of the pulsatile fluid stream at the monitoring location without passing fluid into the pressure transducer. Preferably, a U-bend is provided the first conduit upon its reversal. Alternately, however, a Y-connector may be provided at the turnaround of the first conduit with the third passage of the Y-connector being plugged.
In another aspect of the present invention, there is provided a method for transmitting a positive gauge liquid pressure signal to a gas column from a pulsatile fluid stream at a pressure monitoring location hydrostatically higher than the pressure transducer, comprising the steps of providing a first conduit in communication at one end with the fluid stream at the pressure monitoring location for transmission of the fluid therethrough, providing the first conduit with a predetermined internal diameter of a size sufficient to maintain the meniscus of the fluid intact as the fluid flows through the first conduit, providing a second conduit in communication at one end with the opposite end of the first conduit and in communication at its opposite end with the pressure transducer for containing the gas column, providing the second conduit with a predetermined internal diameter greater than the internal diameter of the first conduit, disposing the first and second conduits such that a portion of the first conduit lies below the pressure transducer and substantially reverses direction before connecting with the second conduit and measuring the pressure of the gas in the second conduit at the pressure transducer to provide a direct measurement of the pressure of the pulsatile fluid stream at the monitoring location without passing fluid into the pressure transducer.
Accordingly, it is a primary object of the present invention to provide novel and improved apparatus and methods for the transmission of a positive gauge liquid pressure signal to a gas column from a pulsatile blood flow stream which is hydrostatically higher than the gas column, while maintaining the gas/blood meniscus.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.